Sample solution: Transfer a quantity of powdered Tablets, equivalent to about 15 mg of zinc, to a centrifuge tube. Add 25 mL of water, sonicate for 1 min, shake, and centrifuge.

Acceptance criteria: Meets the requirements

STRENGTH

• Content of Zinc

Method 1

[NoteA standard stock solution is commercially available at different zinc concentrations, which may be used for preparation of the Standard stock solution. Necessary volumetric adjustment can be made in the Standard solution. Concentrations of the Standard solution and the Sample solution may be modified to fit the linear or working range of the instrument. ]

Standard stock solution: Dissolve 625 mg of zinc oxide, weighed, and previously ignited to constant weight, in 10 mL of nitric acid, and add water to make 500.0 mL. This solution contains 1000 mg/L of zinc.

Standard solution: To a 500-mL volumetric flask add 200 mL of water and 10 mL of nitric acid, and mix thoroughly. Pipette 10.0 mL of the Standard stock solution into the volumetric flask, and dilute with water to volume to obtain a solution having a known concentration of about 20 mg/L of zinc.

Sample solution: Weigh and finely powder NLT 20 Tablets. Transfer weighed portion of the powdered Tablets, equivalent to about 0.1 g of zinc, to a 50-mL flask. Add 10 mL of nitric acid, and heat the solution on a hot plate to boil gently, during which process fuming evolves. Boil the solution for an additional 30 min with constant swirling, during which no fuming should be observed. Cool the solution to room temperature, quantitatively transfer all of the solution to a 500-mL volumetric flask, dilute with water to volume, and mix. Pipette 25.0 mL of this solution into a 250-mL volumetric flask, add 5 mL of nitric acid, dilute with water to volume, mix, and filter.

Analytical wavelength: 206.20 nm. [NoteThe operating conditions may be developed and optimized based on the manufacturer's recommendation. A typical setting includes radio frequency (RF) power of about 1300 watts, argon torch flow of about 15 L/min, argon auxiliary flow of about 0.2 L/min, and a nebulizer flow rate of about 0.8 L/min. ]

Blank: 2% nitric acid solution

Analysis

Samples: Standard solutions, Sample solution, and Blank

Calculate the percentage of the labeled amount of zinc (Zn) in the portion of Tablets taken:

Result = (rU/rS) × (CS/CU) × 100

rU

=

= response from the Sample solution

rS

=

= response from the Standard solution

CS

=

= concentration of the Standard solution (mg/L)

CU

=

= nominal concentration of zinc in the Sample solution (mg/L)

Acceptance criteria: 90.0%110.0%

Method 2

Standard stock solution A: 1000 µg/mL of zinc from zinc oxide in 5 M hydrochloric acid (3.89 mg/mL) and diluted with water to final volume. [NoteDissolve in 5 M hydrochloric acid by warming, if necessary. Cool, and then dilute to final volume. ]

Sample solution: Finely powder NLT 20 tablets. Transfer an equivalent to 5 tablets to a porcelain crucible. Heat the crucible in a muffle furnace maintained at 550 for 612 h, and cool. Add 60 mL of hydrochloric acid, and boil gently on a hot plate or steam bath for 30 min, intermittently rinsing the inner surface of the crucible with 6 N hydrochloric acid. Cool, and quantitatively transfer the contents of the crucible to a 100-mL volumetric flask. Rinse the crucible with small portions of 6 N hydrochloric acid, and add the rinsings to the flask. Dilute with water to volume, and filter, discarding the first 5 mL of the filtrate. Dilute this solution quantitatively, with 0.125 N hydrochloric acid to obtain a nominal concentration of 2 µg/mL of zinc.

Determine the absorbances of the solutions against the Blank. Plot the absorbances of the Standard solutions versus the concentration, in µg/mL, of zinc, and draw the straight line best fitting the five plotted points. From the graph so obtained, determine the concentration, C, in µg/mL, of zinc in the Sample solution.

Calculate the percentage of the labeled amount of zinc (Zn) in the portion of Tablets taken:

Analysis: Proceed as directed in Method 1 or Method 2 for Strength, making any necessary volumetric adjustments.

Sample solution: If Method 1 is used, pipet 10.0 mL of the filtered pooled solution under test to a 50-mL volumetric flask, and dilute with 2% nitric acid solution to 50 mL. If Method 2 is used, dilute the filtered pooled solution under test with 0.125 N hydrochloric acid to a concentration falling within the range of the Standard solutions.

Calculate the percentage of the labeled amount of zinc (Zn) dissolved:

Result = C × (VM/a) × (D/L) × 100

C

=

= concentration of zinc in the Sample solution (mg/L)

VM

=

= volume of Medium, 900 mL

a

=

= aliquot of solution under test (mL)

D

=

= dilution factor to prepare the Sample solution from the aliquot taken